Various principles and methods for determining the desired distances are known in the field of optical and/or optoelectronic distance measurement. One approach is to emit pulsed electromagnetic radiation, for example, laser light, toward a target to be measured and subsequently to receive an echo from this target as the backscattering object, wherein the distance to the target to be measured can be determined on the basis of the runtime of the pulse. Such pulse runtime meters (ToF, i.e., time-of-flight) have prevailed in many fields as standard solutions over time.
Two different approaches are used for detection of the returning pulse.
In the so-called threshold value method, a light pulse is detected when the intensity of the radiation incident on a detector of the distance meter used exceeds a specific threshold value, wherein this is usually implemented with the aid of a comparator stage (i.e., a comparator). This threshold value prevents noise and interfering signals from the background from being detected incorrectly as a useful signal, i.e., as backscattered light of the emitted pulse.
One disadvantage of the threshold value method is that, for example, in the event of weak backscattered pulses, as are caused, for example, by greater measurement distances, a detection is no longer possible if the pulse intensity falls below the detection threshold, i.e., below the threshold value.
A further disadvantage is either the complex structure and/or the lack of precision with respect to the time determination of the comparator signal (i.e., excessively low resolution) of many known time-to-digital converters (TDCs) typically used up to this point in the scope of the threshold value method.
The other approach is based on the sampling of the backscattered pulse (WFD method, waveform digitization). A returning signal is thus detected in this case, the radiation acquired by a detector is sampled, the pulse is identified within the sampled range, and finally the location thereof is determined chronologically with high precision. By using a sufficient number of sampled values and/or summation of the received signal which is synchronous with the emission rate or the coding sequence of the transmitted signal, a useful signal can also be identified under unfavorable circumstances, so that greater distances or background scenarios which are noisy or subject to interference can also be managed. Digital phase measurement is a subvariant of this measurement method.
The limited linear modulation range of the electronic receiver circuit is also problematic in this so-called waveform digitization method (WFD method). At close range, the signal can saturate the receiver, so that the shape of the transmitted signal is no longer correctly ascertained and the runtime is determined with insufficient accuracy.
It is described in WO 2008/009387 in this case for pulse runtime measurements (ToF, Time-of-Flight) that alternatively—i.e., depending on which signal dynamic range of the receiver is addressed by the returning signal—either the threshold value method (with strong returning signal) or the sampling waveform digitization method WFD (with weaker returning signal) can be used.
The inadequate comparability of cross-method determined distance values and the additional structural and/or computer expenditure to effectively provide two completely separate and different circuits (or at least channels) for a distance meter and—with regard to the calibration and finally the calculation of the desired distance value—to have to follow two completely different methods have proven to be disadvantages of this combination of different methods (depending on which signal dynamic range is addressed).